1. Field of the Invention
The present invention relates to a frequency modulation circuit which is used in communication apparatuses such as mobile phones, wireless LAN devices, or the like, a transmission circuit, and a communication apparatus. More particularly, the present invention relates to a frequency modulation circuit which reduces noise present in a desired frequency band and operates with low distortion and high efficiency, a transmission circuit using the frequency modulation circuit, and a communication apparatus.
2. Description of the Background Art
Communication apparatuses such as mobile phones, wireless LAN devices, or the like need to operate with low power consumption while maintaining the accuracy of an output signal. Such communication apparatuses require a frequency modulation circuit which outputs a frequency modulation signal having low distortion and operates with high efficiency and a transmission circuit using the frequency modulation circuit. The following will describe a conventional frequency modulation circuit.
As an example of a conventional frequency modulation circuit, there exists a frequency modulation circuit which is disclosed in Wendell B. Sander, et al, “Polar Modulator for Multi-mode Cell Phones”, US, Tropian, Inc. FIG. 13 is a block diagram showing an exemplarily configuration of a conventional frequency modulation circuit 500. As shown in FIG. 13, the conventional frequency modulation circuit 500 includes first and second arithmetic units 507 and 508, a loop filter 502, a voltage controlled oscillator (VCO) 503, a frequency digital converter (FDC) 504, a DA converter (DAC) 505, and a low pass filter (LPF) 506.
A modulation signal is inputted to the conventional frequency modulation circuit 500 through an input terminal. The modulation signal is inputted to the VCO 503 through the first arithmetic unit 507, the second arithmetic unit 508 and the loop filter 502. The modulation signal is also inputted to the VCO 503 through the DAC 505 and the LPF 506. The VCO 503 frequency-modulates the modulation signal by controlling an oscillatory frequency according to the inputted modulation signal, and outputs a resultant signal as a frequency modulation signal.
The second arithmetic unit 508 adds or subtracts a constant to or from the inputted modulation signal to control the center frequency of the modulation signal. FDC 504 converts the frequency of the frequency modulation signal outputted by the VCO 503 into a digital value, and outputs the converted digital value to the first arithmetic unit 507. The first arithmetic unit 507 adds the digital value outputted by the FDC 504 to the modulation signal outputted by the second arithmetic unit 508, and outputs a resultant signal to the loop filter 502. The loop filter 502 suppresses the high-frequency component of the modulation signal outputted by the first arithmetic unit 507. In other words, the FDC 504, the first arithmetic unit 507, and the loop filter 502 constitute a feedback loop which stabilizes the frequency of the frequency modulation signal outputted by the VCO 503.
Meanwhile, the DAC 505 converts the inputted modulation signal into an analog signal. The LPF 506 serves to suppress noise such as quantization noise which is generated with the processing by the DAC 505.
FIG. 14 illustrates a problem of the conventional frequency modulation circuit 500. FIG. 14 shows a relationship between noise and a frequency which are included in each of an output signal Ax of the DAC 505, an output signal Bx of the LPF 506 when the LPF 506 is not connected to an input of the VCO 503, and an input signal Cx to VCO 503 when the LPF 506 is connected to the input of the VCO 503. It is noted that the symbols Ax to Cx in FIG. 14 correspond to the points Ax to Cx in FIG. 13, respectively.
As shown in FIG. 14, the output signal Ax of the DAC 505 includes substantially uniform quantization noise and the like. The LPF 506 removes noise of a high frequency region from the output signal Ax of the DAC 505. Since the LPF 506 is connected to the input of the VCO 503, the output signal Bx of the LPF 506 is inputted as the input signal Cx to the VCO 503 after noise in the vicinity of a direct current component DC is removed by the operation of the feedback loop of the conventional frequency modulation circuit 500.
As shown in FIG. 14, however, the noise of the input signal Cx to the VCO 503 is not always sufficiently reduced in a frequency band which requires low noise (the shaded area in FIG. 14). Therefore, the conventional frequency modulation circuit 500 has a problem that it cannot output a frequency modulation signal the noise of which is sufficiently reduced in the frequency band which requires low noise.
There is a possibility that in the case where the conventional frequency modulation circuit 500 is installed in a communication apparatus which performs transmission and reception concurrently, the noise generated at the conventional frequency modulation circuit 500 overlaps with a receiving band of the communication apparatus, and this adversely affects the receiving quality of the communication apparatus.